In one commercially available prior art plasma arc torch design, the nozzle assembly includes a nozzle base fabricated from copper or copper alloy and a lower nozzle member fabricated from a ceramic material. The lower nozzle member is glued onto the nozzle base. Both the nozzle base and the lower nozzle member include a bore aligned longitudinally with the longitudinal axis defined by the electrode. An electric arc created by the electrode extends from the discharge end of the electrode through the bores to a workpiece located below the lower nozzle member, while a vortical flow of gas generated between the electrode and the nozzle base creates a plasma flow outwardly through the bores and to the workpiece. An annular water passage is defined between the nozzle base and the lower nozzle member. A jet of water introduced into the passage in surrounding relation to the plasma arc constricts the plasma for better torch operation.
A ceramic composition for the lower nozzle member is desirable in this prior art plasma arc torch because during cutting, the ceramic provides protection from double arcing and insulates the nozzle assembly from heat and plasma generated during torch operation. For example, during cutting, the operator may accidentally move the lower nozzle member into contact with the workpiece. If the lower nozzle member here formed of a metallic material, the torch would be grounded resulting in arc failure as well as possible heat damage.
Additionally, the ceramic composition is desirable to prevent double arcing from the nozzle assembly onto the metallic cup shield mounted on the torch body. The cup includes a forward end having a lip engaging a shoulder on the lower nozzle member. The cup retains the lower nozzle member and the nozzle base in position. Typically the cup is at a potential lying between the electrode and the work. Without the benefit of the ceramic lower nozzle to insulate the cup, there is a larger likelihood that the arc will jump onto the cup.
Although the ceramic lower nozzle member is advantageous because it insulates and resists arcing, a lower nozzle member formed of a ceramic material has several disadvantages. Ceramic materials are difficult to machine or form into high precision parts at a reasonable cost. If close tolerances are desired, expensive forming, machining and fabrication techniques must be adapted. Unless these expensive machining, forming and fabrication techniques are adapted, The desired concentricity and precision of the lower ceramic nozzle member cannot be obtained.
As a result, often during the volume manufacture of nozzle parts, the lower nozzle member has an undesired eccentricity, and the spacing between the lower nozzle member and the nozzle base is inconsistent forming an eccentric, imprecise water passage. The eccentricity in the water passage creates an irregular water spray pattern during torch operation, resulting in ripples forming on the cut surface and beveled cut edges varying in a cut angle.
Additionally, ceramic parts are not well adapted for close tolerance interference fits. Thus, as in the above prior art torches, the ceramic lower nozzle must be glued onto the nozzle base. This low tolerance gluing is not as preferred as securing of the members by the close tolerance interference fits commonly used in metal-to-metal interfaces. Also, ceramic parts typically have poor surface finishes that create irregularities in water spray patterns.